The electrochemical behavior of well-defined Pt-modified Ru(0001) electrodes in perchloric acid solution was investigated by cyclic voltammetry. The Ru(0001) samples were prepared under ultrahigh vacuum conditions and modified by pseudomorphic Pt monolayer islands, which were produced by Pt vapor deposition. The resulting electrodes were quantitatively analyzed by high-resolution scanning tunneling microscopy. The data show that hydrogen adsorption on the strained Pt monolayer is very weak, much weaker than on bulk Pt, which agrees well with theoretical predictions and recent findings for gas-phase hydrogen adsorption. The appearance of a new pair of peaks and the simultaneous disappearance of the peaks characteristic for H adsorption/desorption on the unmodified Ru(0001) electrode in the cyclovoltammogram in the presence of already small amounts of Pt deposit is explained by a catalytic enhancement of the adsorption and desorption of hydrogen on the surrounding Ru substrate areas. It involves hydrogen adsorption on the Pt monolayer islands, followed by removal of the OHad Species at the perimeter of the Pt islands by reaction with H-ad and finally spillover of H-ad to the surrounding Ru substrate in the cathodic scan or the reverse process in the anodic scan. Possible explanations for the physical origin of this novel effect are discussed.